How much fertilizer do you need?

How do I calculate how much fertilizer I need?

Decide how much nitrogen to apply — usually 1 pound per 1,000 square feet — and divide by the share of nitrogen in the bag, the first number of the N-P-K. With a 32-0-4 lawn food that is 1 ÷ 0.32 = 3.1 pounds of product per 1,000 square feet; a 5,000 square foot lawn takes about 15.6 pounds.

What do the three numbers on a fertilizer bag mean?

They are the percentages of nitrogen (N), phosphate (P) and potash (K) by weight. A 50 lb bag of 10-10-10 holds 5 pounds of nitrogen, 5 of phosphate and 5 of potash; the rest is filler that helps spread it evenly. Lawns mostly need nitrogen, so lawn foods have a high first number.

How much nitrogen should I put on my lawn?

University extension services advise no more than 1 pound of nitrogen per 1,000 square feet at a time, and half that for quick-release fertilizer in summer heat. Cool-season lawns are fed mostly in fall, warm-season lawns from late spring through summer; a soil test tells you whether phosphorus and potassium are needed at all.

How do I set my spreader?

Look for the spreader chart on the bag — it lists settings for common spreaders by the rate you want. The calculator shows that rate as pounds of product per 1,000 square feet. For even coverage, apply half at a time in two directions at right angles.

How many bags of fertilizer do I need?

Divide the pounds of product by the weight of a bag and round up. For 15.6 pounds, one 25 lb bag or two 10 lb bags. Many bags also print how many square feet they cover — at their own rate, which may differ from yours.

Worked example: a 6,500 square foot lawn

At 1 pound of nitrogen per 1,000 square feet, 6,500 square feet needs 6.5 pounds of nitrogen. With 32-0-4 lawn food (32% nitrogen) the product weight is 6.5 ÷ 0.32 = 20.3 pounds, which is one 25 lb bag with a little left over. With 46-0-0 urea the same nitrogen takes only 14.1 pounds, and with 10-10-10 it takes 65.0 pounds, more than a 50 lb bag, which also puts down the same weight of phosphate and potash that the lawn may not need. The nitrogen is identical in all three cases; only the carrier differs.

How much area does one bag cover?

Turn the calculation around: pounds in the bag × the nitrogen share = pounds of nitrogen in the bag, and dividing that by the rate per 1,000 square feet gives the area in thousands of square feet. A 25 lb bag of 32-0-4 holds 8 pounds of nitrogen, so at 1 pound per 1,000 square feet it covers 8,000 square feet, and at half a pound it covers 16,000. This is how a printed coverage figure on a bag is derived, so check which rate the bag assumed.

What do the metric units mean?

One pound per 1,000 square feet is 4.88 grams of nitrogen per square meter (453.592 grams in a pound, 92.903 square meters in 1,000 square feet). The calculator's metric mode applies the same rule of nitrogen rate divided by the nitrogen share, with kilograms of product.

Common fertilizer-math mistakes

Treating the bag weight as the nitrogen weight: a 50 lb bag of 10-10-10 contains 5 pounds of nitrogen, not 50. Using the percentage as a whole number in the formula (dividing by 32 rather than 0.32). Reusing a spreader setting from another product: urea at 46% nitrogen needs less than half the product weight of a 20% fertilizer for the same nitrogen, so the setting does not carry over from one bag to another. Spreading by the area of the whole property rather than the lawn alone, which counts the house, drive and beds. Applying the whole amount in one pass instead of splitting it in two crossing passes, which tends to leave stripes. Local rules on phosphorus and on application dates exist in some places, so check them before buying.

Sources and related calculators

Sources: University of New Hampshire Extension, “Calculating Lawn Fertilizer Rates” (pounds of nitrogen in a bag and the area it covers); Rutgers NJAES fact sheet FS839 and Iowa State University Extension for the nitrogen-per-application guidance. Related: grass seed calculator, sod calculator, lawn watering calculator and topsoil calculator.